Quantum Computing Wins a Nobel

Earlier this week, Serge Haroche, a Frenchman, and David J. Wineland, an American, were awarded the Nobel Prize in Physics “for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems”—whatever that means. Even if you’re interested in quantum computers, the machines that Haroche and Wineland’s discoveries are helping to build, you’re likely to find that statement confusing. Both men work in the field of quantum optics; that is, they figure out ways to look at very small particles, like photons, and determine their quantum states. But wait—isn’t the whole point of quantum mechanics that you can’t know the states of individual particles? Aren’t they actually waves? Or probabilities? Or cats? (In boxes?) By now, your own mind may have a entered a quantum state: It is now knowledgeable and ignorant simultaneously.

Quantum computing is one of the most interesting things happening in science today, but it’s also one of the hardest to talk about. As its laudably concrete name suggests, quantum computing combines two fields of scientific inquiry: quantum physics, which many people already struggle to understand (by reading books by Brian Greene, Michio Kaku, and others); and computing, which, because this year is the centenary of Alan Turing, is finally starting to get a little attention. Essentially, the goal is to build an incredibly small computer. As even high-school students know nowadays, the laws of physics that govern normal-sized objects differ from the laws that govern very, very small ones. Build something on the scale of a circuit-board, and it’s governed by one set of laws; build something on the scale of an atom, and it’s governed by another. The “logic gates” in the computer on your desk obey the same physical laws as you and me. But a super-small logic gate—one that consisted, say, of a single photon—would obey different, quantum-mechanical laws. These laws are so weird that, for decades, physicists, including Einstein, balked at accepting them. Now, the physicists, mathematicians, and computer scientists who work on quantum computing think that they might provide a kind of computational wiggle-room. A computer built on that scale might be able to solve problems that today’s computers cannot solve. (A good deal of the research into quantum computing is funded by governments hoping to break codes that, for all practical purposes, are unbreakable with today’s computers.)

In May of last year, Rivka Galchen wrote an article about quantum computing for The New Yorker. Her piece, “Dream Machine,” focusses on the physicist David Deutsch, who many people think first came up with the idea for a quantum computer. The idea, Deutsch explains, emerged because of a debate he was having with a colleague, Charles Bennett, a scientist at I.B.M., about a sub-topic in computer science called computational-complexity theory. Computational-complexity theory is a way of thinking about why some problems are relatively simple to solve or, more properly, “compute,” while others are, for all practical purposes, impossible. (The simplest example is “prime factorization,” the mathematical problem around which many codes are built: Take two prime numbers, and it’s easy to compute their product; take a big number and try to find its prime factors, and it could take billions of years.) Deutsch recalls objecting to the whole premise of the theory. Surely, he argued, it’s meaningless to refer to a problem as “hard” or “easy” to compute, because the problem’s hardness or easiness depends upon the computer you’re using. There is no such thing as an “absolute” computer.

Deutsch continued, “Then Charlie said, quietly, ‘Well, the thing is, there is a fundamental computer. The fundamental computer is physics itself.’” That impressed Deutsch. Computational complexity [could reference] how complicated a computation was on that most universal computer, that of the physics of the world. “I realized that Charlie was right about that,” Deutsch said. “Then I thought, But these guys are using the wrong physics. They realized that complexity theory was a statement about physics, but they didn’t realize that it mattered whether you used the true laws of physics, or some approximation, i.e., classical physics.” Deutsch began rewriting Turing’s universal-computer work using quantum physics. “Some of the differences are very large,” he said. Thus, at least in Deutsch’s mind, the quantum universal computer was born.”

In case you missed it, quantum computing depends on a very deep idea: that the world itself is a kind of computer. Right now, when we program our computers, we are programming machines that behave in an artificial way, according to the laws we’ve built into them. It’s possible, though, to compute in a much more intimate and natural way—by programming the individual atoms themselves to solve problems using what Deutsch calls “the true laws of physics.” It’s no wonder that some journals rejected his early quantum-computing papers for being too philosophical.

Now, though—as Haroche and Wineland’s Nobel shows—quantum computing is extremely concrete. In her article, Galchen visits a lab at Oxford where scientists are building a quantum computer:

something that resembled an oversize air-hockey table chaotically populated with a specialty Lego set, with what looked like a salad-bar sneeze guard hovering over it; this extended apparatus comprised lasers and magnetic-field generators and optical cavities, all arranged at just the right angles to manipulate and protect from interference the eight tiny qubits housed in a steel tube at the table’s center.

(A “qubit” is quantum computing’s version of a bit.) For some physicists, including Deutsch, a real working quantum computer would give us concrete proof of some of the stranger aspects of quantum theory. Deutsch, for example, thinks that quantum computing, if it works, will provide evidence in favor of the Many Worlds Interpretation of quantum theory—that is, the idea that “anything that is possible in fact is,” in a parallel universe that is just as real as our own. (Other physicists disagree.) Is quantum computing about physics? About computers? About philosophy? In true quantum-theoretical style, it’s about all three simultaneously.

Joshua Rothman is The New Yorker’s archive editor. He is also a frequent contributor to newyorker.com, where he writes a blog about books and ideas.